1. Field of the Invention
The present invention is directed to the field of thermal image detection and more specifically to detectors which provide internal integration of a received image.
2. Description of the Prior Art
The background prior art, with respect to the present invention, is adequately described in U.S. Pat. No. 3,995,159, which is incorporated herein by reference. The thermal imaging detector device described in the above referenced patent, includes a uniform linear elongated strip of semiconductor material, which is rendered photoconductive by the generation of electron and hole photocarriers when a radiation image is scanned along the strip. It further includes means for applying a lengthwise electric bias current through the strip for the purpose of producing an ambipolar drift of the photocarriers in the strip having a drift velocity that matches the image scanning velocity.
The described strip is a rectangularly shaped ribbon which has one surface totally exposed to receive scanned radiation over its length. The image is scanned along the strip at a velocity which is said to match that of the ambipolar drift so that an integrated picture can be built up from all the active parts (accumulated charge packets) that traverse a readout electrode.
We have found that as a packet of charge moves along the exposed surface of the uniform integrating detector strip it tends to bloom during the integration period of the scan which degrades the resolution of the image.
We have also found that the uniform integrating detector strip has a tendency to lose some of the accumulated carriers at the width edges, due to surface recombination. This loss of accumulated carriers reduces the amplitude of the signal readout from the detector.
In the type of detector wherein integration is achieved on the detector itself, the individual pixels of image are scanned across the detector and a corresponding packet of photogenerated minority carriers are generated to flow along the length of the detector at a velocity determined by the bias field. It is important that each packet of charge flows at the same velocity as the scanned image so that when each packet arrives at the readout electrode, it will have a charge density concentration that corresponds to the focused pixel intensity of the image integrated over the period of the scan.
If the velocities are not synchronous, the resultant signal will have a lower amplitude and adjacent signals are less distinctive with a resultant lower resolution.
The above-referenced patent suggests that small variations in the image scanned velocity may be compensated for by adjusting the bias current. That solution may be appropriate where a single pixel is focused onto the detector and a single packet of photocarriers is generated to track that scanned pixel. In that case, any variation between the velocities of the packet and the pixel may be compensated for by appropriately varying the bias current and thereby causing the single packet to increase or decrease its drift velocity in order to track with the scanned pixel.
When a series of pixels are scanned along the detector and are simultaneously present on the detector, a plurality of adjacent photocarrier packets are correspondingly generated on the detector during each scan. Therefore, if one were to vary the bias current to compensate for variations between the scanned velocity of any pixel and packet, such variation in the current will have a simultaneous effect on all the photocarrier packets present on the detector.
We have noted that the drift velocity "V" of any charge packet is directly dependent upon the electric bias field density "E" and a mobility factor "U" (i.e., V=Eu). As long as each factor is constant, the drift velocity V will remain constant. However, the mobility factor u is inversely related to the charge buildup and, for a constant E, causes the drift velocity of each charge packet to decrease as that packet traverses the length of the detector.
Due to the fact that the charge buildup in each packet is primarily due to the background radiation, the charge buildup due to the focused image feature is of minor consideration in viewing the change in drift velocity as each packet travels the length of the detector.